1. Field of the Invention
The present invention relates to a cryogenic cooling apparatus and, more particularly, to a cryogenic cooling apparatus which is suitable for use as split type cryogenic cooling system in which a refrigerator, serving as a cold heat source and a cryostat, serving as cold heat applying device, are separately installed. Still more particularly, the invention is concerned with a cryogenic cooling apparatus of the type mentioned above, wherein the cryostat has at least two stages of heat exchangers and a colder end of each heat exchanger is thermally connected to a shield plate of corresponding temperature and to a vessel, so that the heat input from the outside is reduced to permit reduction in a capacity and size of the refrigerator and also a simplification of construction of the cryogenic cooling apparatus as a whole.
2. Description of the Prior Art
Various types of cryogenic cooling apparatus which make use of helium gas as working gas have been proposed and used, and these conventional cryogenic cooling apparatus can be broadly sorted into a unit type apparatus, in which a refrigerator and a cryostat are constructed as a unit with each other, and a separate type apparatus, in which the refrigerator and the cryostat are constructed separately and connected through communication pipes. Generally, the cryogenic cooling apparatus of small capacity are constructed as the unit type apparatus, whereas, the split type apparatus find their use mainly when the capacity is comparatively large.
The cryogenic cooling apparatus of small capacity, however, suffers from a problem of heavy vibration and large noise because, in most cases, a reciprocating type expansion engine is used in the refrigerator of the cryogenic cooling apparatus of such small capacity. Therefore, in some cases, the split type construction is adopted even in the cryogenic cooling apparatus of small capacity, particularly when it is required to keep the vibration and noise away from the cryostat which serves the cold heat applying device.
FIG. 1 provides an example of a conventional split type cryogenic cooling apparatus having a refrigerator section which includes a first stage compressor 1, second stage compressor 2 and a refrigerator generally designated by the reference numeral 3, and a cryostat generally designated by the reference numeral 14. A communication pipe 26, having a number of working gas pipes, connects the refrigerator 3 and to the cryostat 14. A part of high pressure helium gas, discharged from the second stage compressor 2, is supplied through a valve mechanism 4 to a first expansion engine 5 and a second expansion engine 6 which constitute expansion means, and generates the cold heat through expansion in these engines 5, 6. The helium gas of an intermediate pressure, after the expansion, is returned to the juncture between the first stage compressor 1 and the second stage compressor 2. On the other hand, the remaining part of the helium gas discharged from the second stage compressor 2 is delivered to a first heat exchanger 9 in a cold box 13 and is cooled while flowing through the first heat exchanger 9. The remaining part of helium gas is then introduced through a helium gas pipe 37a, in the communication pipe system 26, into a first shield station 23 provided in the cryostat 13 to cool a first shield plate 15 connected to the first shield station 23 to thereby prevent a heat leak into the cryostat 14. The high pressure helium gas of, coming out of the first shield station 23, is introduced, through a helium gas pipe 37b in the communication pipe system 26, into a first cold station 7, provided on the end of the first expansion engine 5, and is cooled to lower temperature through a heat exchange in the first cold station 7. The helium gas is then delivered to and further cooled by a second heat exchanger 10 and, thereafter, introduced through a helium gas pipe 37c, in the communication pipe system 26, into a second shield station 24 in the cryostat 14 and cools a second shield plate 16. The high temperature helium gas coming from the second shield station 24 is then introduced through a helium gas pipe 37d, in the communication pipe system 26, to a second cold station 8 provided on the end of the second expansion engine 6 so as to be further cooled to lower temperature and sent to a third heat exchanger 11. The high pressure helium gas, which has been sufficiently cooled through heat exchange in the third heat exchanger 11, is introduced through a helium gas pipe 37e, in the communication pipe system 26, to a Joule-Thomson valve 18 in which the helium gas makes an isoenthalpic expansion through a pressure reduction. Consequently, the helium gas is partly liquefied and the liquid fraction is stored in a vessel 17. As a sufficient liquefied helium is accumulated in the vessel 17, the helium gas of low pressure and low temperature, after pressure reduction across the Joule-Thomson valve 18, is introduced into a low-pressure passage of the third heat exchanger 11 in the cold box 13 through a three-way valve 19 and a condenser heat exchanger 20 and through a helium gas pipe 37f in the communication pipe system 26, and is returned to the suction side of the first stage compressor 1 through the low-pressure passages of the second heat exchanger 10 and the first heat exchanger 9. Needless to say, when the helium gas flows through the low-pressure passages through the successive heat exchangers, it is heated through heat exchange with the helium gas flowing through the high-pressure passages, and finally becomes low-pressure helium gas of a substantially room temperature, before it is returned to the suction side of the first stage compressor 1.
A so-called heat pipe 25 containing a fluid which is boiled or condensed permits a heat exchange between the vessel 17 and the second shield plate 16 to promote the cooling of the vessel 17 when the temperature of the vessel 17 is higher than that of the second shield plate 16, during the cooling down, i.e. the start up of the cryogenic cooling apparatus as a whole. The heat exchange through the heat pipe 25 is stopped when the temperature of the vessel 17 has come down below the temperature of the second shield plate 16. The heat pipe in some cases is termed "thermal diode," "thermal coupling" and so forth.
The conventional cryogenic cooling apparatus of FIG. 1 suffers from the following problems. First, since the communication pipe system 26 has a large number of helium gas pipes (six pipes in the illustrated case), the invasion by heat is correspondingly increased to cause a shortage of the refrigerating power or liquefying power particularly in a cryogenic cooling apparatus of small capacity, so that a correspondingly large capacity refrigerator is required for obtaining the desired performance of the cooling apparatus. Additionally, there is a not so negligible heat input through the heat pipe 25 into the vessel 17 which constitutes a low-temperature part. Namely, since the heat pipe 25 consists of a tubular vessel containing the heat exchanging medium and permanently connects the second shield plate 16 and the vessel 17, heat is transferred inconveniently through the wall of the heat pipe 25 even after a heat exchange, through circulation of the fluid, is stopped after a cooling down of the vessel 17 to a temperature equal to that of the second shield plate 16. This phenomenon is equivalent to the heat leak into the vessel 17 which constitutes a low-temperature part of the cryogenic cooling apparatus. Thus, after the cooling of the vessel 17 down to the operating temperature, the heat pipe 25 undesirably constitutes a heat leak into the vessel 17.